The objectives of the proposed work are to identify the subcellular site of action of ryanodine in cardiac muscle and to establish the usefulness of this drug as an effector of sarcoplasmic reticular Ca2+ movements in this tissue. Ryanodine depresses myocardial contractility when present in submicromolar quantities in a species-dependent manner. Based on our previous work and on that of others we have developed as a working hypothesis that ryanodine acts to decrease sarcoplasmic reticular Ca2+ release and consequently the cytoplasmic availability of contractile-Ca2+. We will directly test this heuristic hypothesis as well as alternative possibilities in the studies proposed in the present application. This work will involve investigations of the effects exerted by ryanodine on the Ca2+ transport systems located in sarcolemmal, sarcoplasmic reticular and mitochondrial membranes. We will also assess the ability of this drug to alter the sensitivity of the myofibrillar protein-ATPase to Ca2+. In addition, we will conduct a systematic determination of the electrophysiological changes produced by ryanodine. Finally, we will characterize the binding of 3H-ryanodine to both intact tissues and to isolated subcellular fractions. All of this work will involve comparisons between cat, rabbit and rat ventricular myocardium. The results of these studies will establish the usefulness of ryanodine as an experimental tool with which to prove sarcoplasmic reticular involvement in myocardial E-C coupling, the contributions made by this and other cellular activities to this process and how these contributions differ in cardiac muscle from different species.